When there is no alternative, a real-time program will typically open one of the files named /dev/tty*. The names, and some hardware details, for these devices are documented in the serial(7) reference page. Information specific to two serial adapter boards is in the duart(7) reference page and the cdsio(7) reference page.
When a process opens a serial device, a line discipline STREAMS module is pushed on the stream by default. If the real-time device is not a terminal and doesn't support the usual line controls, this module can be removed. Use the I_POP ioctl (see the streamio(7) reference page) until no modules are left on the stream. This minimizes the overhead of serial input, at the cost of receiving completely raw, unprocessed input.
An important feature of current device drivers for serial ports is that they try to minimize the overhead of handling the many interrupts that result from high character data rates. The serial I/O boards interrupt at least every 4 bytes received, and in some cases on every character (at least 480 interrupts a second, and possibly 1920, at 19,200 bps). Rather than sending each input byte up the stream as it arrives, the drivers buffer a few characters and send multiple characters up the stream.
When the line discipline module is present on the stream, this behavior is controlled by the termio settings, as described in the termio(7) reference page for non-canonical input. However, a real-time program will probably not use the line-discipline module. The hardware device drivers support the SIOC_ITIMER ioctl that is mentioned in the serial(7) reference page, for the same purpose.
The SIOC_ITIMER function specifies the number of clock ticks (see "Tick Interrupts") over which it should accumulate input characters before sending a batch of characters up the input stream. A value of 0 requests that each character be sent as it arrives (do this only for devices with very low data rates, or when it is absolutely necessary to know the arrival time of each input byte). A value of 5 tells the driver to collect input for 5 ticks (50 milliseconds, or as many as 24 bytes at 19,200 bps) before passing the data along.